Neutralization resistance of virological synapse-mediated HIV-1 Infection is regulated by the gp41 cytoplasmic tail

Engineered CD4 T cells expressing a membrane anchored viral inhibitor restrict HIV-1 through cis and trans mechanisms

HIV-1 infection of target cells can occur through either cell-free virions or cell-cell transmission in a virological synapse, with the latter mechanism of infection reported to be 100- to 1,000-fold more efficient. Neutralizing antibodies and entry inhibitors effectively block cell-free HIV-1, but with few exceptions, they display much less inhibitory activity against cell-mediated HIV-1 transmission. Previously, we showed that engineering HIV-1 target cells by genetically linking single-chain variable fragments (scFvs) of antibodies to glycosyl phosphatidylinositol (GPI) potently blocks infection by cell-free virions and cell-mediated infection by immature dendritic cell (iDC)-captured HIV-1. Expression of scFvs on CD4+ cell lines by transduction with X5 derived anti-HIV-1 Env antibody linked to a GPI attachment signal directs GPI-anchored scFvs into lipid rafts of the plasma membrane. In this study, we further characterize the effect of GPI-scFv X5 on cell-cell HIV-1 transmission from DCs to target cells. We report that expression of GPI-scFv X5 in transduced CD4+ cell lines and human primary CD4+ T cells potently restricts viral replication in iDC- or mDC-captured HIV-1 in trans. Using live-cell imaging, we observed that when GPI-GFP or GPI-scFv X5 transduced T cells are co-cultured with iDCs, GPI-anchored proteins enrich in contact zones and subsequently migrate from T cells into DCs, suggesting that transferred GPI-scFv X5 interferes with HIV-1 infection of iDCs. We conclude that GPI-scFv X5 on the surface of transduced CD4+ T cells not only potently blocks cell-mediated infection by DCs, but it transfers from transduced cells to the surface of iDCs and neutralizes HIV-1 replication in iDCs. Our findings have important implications for HIV-1 antibody-based immunotherapies as they demonstrate a viral inhibitory effect that extends beyond the transduced CD4+ T cells to iDCs which can enhance HIV-1 replication.

The HIV Env Glycoprotein Conformational States on Cells and Viruses

The HIV Env glycoprotein is the surface glycoprotein responsible for viral entry into CD4+ immune cells. During infection, Env also serves as a primary target for antibody responses, which are robust but unable to control virus replication. Immune evasion by HIV-1 Env appears to employ complex mechanisms to regulate what antigenic states are presented to the immune system. Immunodominant features appear to be distinct from epitopes that interfere with Env functions in mediating infection. Further, cell-cell transmission studies indicate that vulnerable conformational states are additionally hidden from recognition on infected cells, even though the presence of Env at the cell surface is required for viral infection through the virological synapse. Cell-cell infection studies support that Env on infected cells is presented in distinct conformations from that on virus particles. Here we review data regarding the regulation of conformational states of Env and assess how regulated sorting of Env within the infected cell may underlie mechanisms to distinguish Env on the surface of virus particles versus Env on the surface of infected cells. These mechanisms may allow infected cells to avoid opsonization, providing cell-to-cell infection by HIV with a selective advantage during evolution within an infected individual. Understanding how distinct Env conformations are presented on cells versus viruses may be essential to designing effective vaccine approaches and therapeutic strategies to clear infected cell reservoirs.

Single-chain variable fragments of broadly neutralizing antibodies prevent HIV cell-cell transmission

Broadly neutralizing antibodies (bNAbs) are able to prevent HIV infection following passive administration. Single-chain variable fragments (scFv) may have advantages over IgG as their smaller size permits improved diffusion into mucosal tissues. We have previously shown that scFv of bNAbs retain significant breadth and potency against cell-free viral transmission in a TZM-bl assay. However, scFv have not been tested for their ability to block cell-cell transmission, a model in which full-sized bNAbs lose potency. We tested 4 scFv (CAP256.25, PGT121, 3BNC117 and 10E8v4) compared to IgG, in free-virus and cell-cell neutralization assays in A3.01 cells, against a panel of seven heterologous viruses. We show that free-virus neutralization titers in the TZM-bl and A3.01 assays were not significantly different, and confirm that scFv show a 1 to 32-fold reduction in activity in the cell-free model, compared to IgG. However, whereas IgG show 3.4 to 19-fold geometric mean potency loss in cell-cell neutralization compared to free-virus transmission, scFv had more comparable activity in the two assays, with only a 1.3 to 2.3-fold reduction. Geometric mean IC₅₀ of scFv for cell-cell transmission ranged from 0.65 μg/ml (10E8v4) to 2.3 μg/ml (3BNC117) with IgG and scFv neutralization showing similar potency against cell-associated transmission. Therefore, despite the reduced activity of scFv in cell-free assays, their retention of activity in the cell-cell format may make scFv useful for the prevention of both modes of transmission in HIV prevention studies. Importance Broadly neutralizing antibodies (bNAbs) are a major focus for passive immunization against HIV, with the recently concluded HVTN AMP (Antibody Mediated Protection) trial providing proof of concept. Most studies focus on cell-free HIV, however cell-associated virus may play a significant role in HIV infection, pathogenesis and latency. Single-chain variable fragments (scFv) of antibodies may have increased tissue penetration, and reduced immunogenicity. We previously demonstrated that scFv of four HIV-directed bNAbs (CAP256-VRC26.25, PGT121, 3BNC117 and 10E8v4) retain significant potency and breadth against cell-free HIV. As some bNAbs have been shown to lose potency against cell-associated virus, we investigated the ability of bNAb scFv to neutralize this mode of transmission. We demonstrate that unlike IgG, scFv of bNAbs are able to neutralize cell-free and cell-associated virus with similar potency. These scFv, which show functional activity in the therapeutic range, may therefore be suitable for further development as passive immunity for HIV prevention.

Advancing HIV Broadly Neutralizing Antibodies: From Discovery to the Clinic

Despite substantial progress in confronting the global HIV-1 epidemic since its inception in the 1980s, better approaches for both treatment and prevention will be necessary to end the epidemic and remain a top public health priority. Antiretroviral therapy (ART) has been effective in extending lives, but at a cost of lifelong adherence to treatment. Broadly neutralizing antibodies (bNAbs) are directed to conserved regions of the HIV-1 envelope glycoprotein trimer (Env) and can block infection if present at the time of viral exposure. The therapeutic application of bNAbs holds great promise, and progress is being made toward their development for widespread clinical use. Compared to the current standard of care of small molecule-based ART, bNAbs offer: (1) reduced toxicity; (2) the advantages of extended half-lives that would bypass daily dosing requirements; and (3) the potential to incorporate a wider immune response through Fc signaling. Recent advances in discovery technology can enable system-wide mining of the immunoglobulin repertoire and will continue to accelerate isolation of next generation potent bNAbs. Passive transfer studies in pre-clinical models and clinical trials have demonstrated the utility of bNAbs in blocking or limiting transmission and achieving viral suppression. These studies have helped to define the window of opportunity for optimal intervention to achieve viral clearance, either using bNAbs alone or in combination with ART. None of these advances with bNAbs would be possible without technological advancements and expanding the cohorts of donor participation. Together these elements fueled the remarkable growth in bNAb development. Here, we review the development of bNAbs as therapies for HIV-1, exploring advances in discovery, insights from animal models and early clinical trials, and innovations to optimize their clinical potential through efforts to extend half-life, maximize the contribution of Fc effector functions, preclude escape through multiepitope targeting, and the potential for sustained delivery.

NMR Model of the Entire Membrane-Interacting Region of the HIV-1 Fusion Protein and Its Perturbation of Membrane Morphology

HIV-1 envelope glycoprotein (Env) is a transmembrane protein that mediates membrane fusion and viral entry. The membrane-interacting regions of the Env, including the membrane-proximal external region (MPER), the transmembrane domain (TMD), and the cytoplasmic tail (CT), not only are essential for fusion and Env incorporation but also can strongly influence the antigenicity of the Env. Previous studies have incrementally revealed the structures of the MPER, the TMD, and the KS-LLP2 regions of the CT. Here, we determined the NMR structure of the full-length CT using a protein fragment comprising the TMD and the CT in bicelles that mimic a lipid bilayer, and by integrating the new NMR data and those acquired previously on other gp41 fragments, we derived a model of the entire membrane-interacting region of the Env. The structure shows that the CT forms a large trimeric baseplate around the TMD trimer, and by residing in the headgroup region of the lipid bilayer, the baseplate causes severe exclusion of lipid in the cytoleaflet of the bilayer. All-atom molecular dynamics simulations showed that the overall structure of the MPER-TMD-CT can be stable in a viral membrane and that a concerted movement of the KS-LLP2 region compensates for the lipid exclusion in order to maintain both structure and membrane integrity. Our structural and simulation results provide a framework for future research to manipulate the membrane structure to modulate the antigenicity of the Env for vaccine development and for mutagenesis studies for investigating membrane fusion and Env interaction with the matrix proteins.

Mutations that confer resistance to broadly-neutralizing antibodies define HIV-1 variants of transmitting mothers from that of non-transmitting mothers

Despite considerable reduction of mother-to-child transmission (MTCT) of HIV through use of maternal and infant antiretroviral therapy (ART), over 150,000 infants continue to become infected with HIV annually, falling far short of the World Health Organization goal of reaching <20,000 annual pediatric HIV cases worldwide by 2020. Prior to the widespread use of ART in the setting of pregnancy, over half of infants born to HIV-infected mothers were protected against HIV acquisition. Yet, the role of maternal immune factors in this protection against vertical transmission is still unclear, hampering the development of synergistic strategies to further reduce MTCT. It has been established that infant transmitted/founder (T/F) viruses are often resistant to maternal plasma, yet it is unknown if the neutralization resistance profile of circulating viruses predicts the maternal risk of transmission to her infant. In this study, we amplified HIV-1 envelope genes (env) by single genome amplification and produced representative Env variants from plasma of 19 non-transmitting mothers from the U.S. Women Infant Transmission Study (WITS), enrolled in the pre-ART era. Maternal HIV Env variants from non-transmitting mothers had similar sensitivity to autologous plasma as observed for non-transmitting variants from transmitting mothers. In contrast, infant variants were on average 30% less sensitive to paired plasma neutralization compared to non-transmitted maternal variants from both transmitting and non-transmitting mothers (p = 0.015). Importantly, a signature sequence analysis revealed that motifs enriched in env sequences from transmitting mothers were associated with broadly neutralizing antibody (bnAb) resistance. Altogether, our findings suggest that circulating maternal virus resistance to bnAb-mediated neutralization, but not autologous plasma neutralization, near the time of delivery, predicts increased MTCT risk. These results caution that enhancement of maternal plasma neutralization through passive or active vaccination during pregnancy may potentially drive the evolution of variants fit for vertical transmission.

Despite widespread, effective use of ART among HIV infected pregnant women, new pediatric HIV infections increase by about 150,000 every year. Thus, alternative strategies will be required to reduce MTCT and eliminate pediatric HIV infections. Interestingly, in the absence of ART, less than half of HIV-infected pregnant women will transmit HIV, suggesting natural immune protection of infants from virus acquisition. To understand the impact of maternal plasma autologous virus neutralization responses on MTCT, we compared the plasma and bnAb neutralization sensitivity of the circulating viral population present at the time of delivery in untreated, HIV-infected transmitting and non-transmitting mothers. While there was no significant difference in the ability of transmitting and non-transmitting women to neutralize their own circulating virus strains, specific genetic motifs enriched in variants from transmitting mothers were associated with resistance to bnAbs, suggesting that acquired bnAb resistance is a common feature of vertically-transmitted variants. This work suggests that enhancement of plasma neutralization responses in HIV-infected mothers through passive or active vaccination could further drive selection of variants that could be vertically transmitted, and cautions the use of passive bnAbs for HIV-1 prophylaxis or therapy during pregnancy.

Design, synthesis, and antiviral activity of a series of CD4-mimetic small-molecule HIV-1 entry inhibitors

We presented our continuing stride to optimize the second-generation NBD entry antagonist targeted to the Phe43 cavity of HIV-1 gp120. We have synthesized thirty-eight new and novel analogs of NBD-14136, earlier designed based on a CH₂OH "positional switch" hypothesis, and derived a comprehensive SAR. The antiviral data confirmed that the linear alcohol towards the "N" (C4) of the thiazole ring yielded more active inhibitors than those towards the "S" (C5) of the thiazole ring. The best inhibitor, NBD-14273 (compound 13), showed both improved antiviral activity and selectivity index (SI) against HIV-1_HXB2 compared to NBD-14136. We also tested NBD-14273 against a large panel of 50 HIV-1 Env-pseudotyped viruses representing clinical isolates of diverse subtypes. The overall mean data indicate that antiviral potency against these isolates improved by ~3-fold, and SI also improved ~3-fold compared to NBD-14136. This new and novel inhibitor is expected to pave the way for further optimization to a more potent and clinically relevant inhibitor against HIV-1.

Mechanistic Analysis of the Broad Antiretroviral Resistance Conferred by HIV-1 Envelope Glycoprotein Mutations

Despite the effectiveness of antiretroviral (ARV) therapy, virological failure can occur in some HIV-1-infected patients in the absence of mutations in drug target genes. We previously reported that, in vitro, the lab-adapted HIV-1 NL4-3 strain can acquire resistance to the integrase inhibitor dolutegravir (DTG) by acquiring mutations in the envelope glycoprotein (Env) that enhance viral cell-cell transmission. In this study, we investigated whether Env-mediated drug resistance extends to ARVs other than DTG and whether it occurs in other HIV-1 isolates. We demonstrate that Env mutations can reduce susceptibility to multiple classes of ARVs and also increase resistance to ARVs when coupled with target-gene mutations. We observe that the NL4-3 Env mutants display a more stable and closed Env conformation and lower rates of gp120 shedding than the WT virus. We also selected for Env mutations in clinically relevant HIV-1 isolates in the presence of ARVs. These Env mutants exhibit reduced susceptibility to DTG, with effects on replication and Env structure that are HIV-1 strain dependent. Finally, to examine a possible in vivo relevance of Env-mediated drug resistance, we performed single-genome sequencing of plasma-derived virus from five patients failing an integrase inhibitor-containing regimen. This analysis revealed the presence of several mutations in the highly conserved gp120-gp41 interface despite low frequency of resistance mutations in integrase. These results suggest that mutations in Env that enhance the ability of HIV-1 to spread via a cell-cell route may increase the opportunity for the virus to acquire high-level drug resistance mutations in ARV target genes.IMPORTANCE Although combination antiretroviral (ARV) therapy is highly effective in controlling the progression of HIV disease, drug resistance can be a major obstacle. Recent findings suggest that resistance can develop without ARV target gene mutations. We previously reported that mutations in the HIV-1 envelope glycoprotein (Env) confer resistance to an integrase inhibitor. Here, we investigated the mechanism of Env-mediated drug resistance and the possible contribution of Env to virological failure in vivo We demonstrate that Env mutations can reduce sensitivity to major classes of ARVs in multiple viral isolates and define the effect of the Env mutations on Env subunit interactions. We observed that many Env mutations accumulated in individuals failing integrase inhibitor therapy despite a low frequency of resistance mutations in integrase. Our findings suggest that broad-based Env-mediated drug resistance may impact therapeutic strategies and provide clues toward understanding how ARV-treated individuals fail therapy without acquiring mutations in drug target genes.

Elucidating the Basis for Permissivity of the MT-4 T-Cell Line to Replication of an HIV-1 Mutant Lacking the gp41 Cytoplasmic Tail

HIV-1 encodes an envelope glycoprotein (Env) that contains a long cytoplasmic tail (CT) harboring trafficking motifs implicated in Env incorporation into virus particles and viral transmission. In most physiologically relevant cell types, the gp41 CT is required for HIV-1 replication, but in the MT-4 T-cell line the gp41 CT is not required for a spreading infection. To help elucidate the role of the gp41 CT in HIV-1 transmission, in this study, we investigated the viral and cellular factors that contribute to the permissivity of MT-4 cells to gp41 CT truncation. We found that the kinetics of HIV-1 production and virus release are faster in MT-4 than in the other T-cell lines tested, but MT-4 cells express equivalent amounts of HIV-1 proteins on a per-cell basis relative to cells not permissive to CT truncation. MT-4 cells express higher levels of plasma-membrane-associated Env than nonpermissive cells, and Env internalization from the plasma membrane is less efficient than that from another T-cell line, SupT1. Paradoxically, despite the high levels of Env on the surface of MT-4 cells, 2-fold less Env is incorporated into virus particles produced from MT-4 than SupT1 cells. Contact-dependent transmission between cocultured 293T and MT-4 cells is higher than in cocultures of 293T with most other T-cell lines tested, indicating that MT-4 cells are highly susceptible to cell-to-cell infection. These data help to clarify the long-standing question of how MT-4 cells overcome the requirement for the HIV-1 gp41 CT and support a role for gp41 CT-dependent trafficking in Env incorporation and cell-to-cell transmission in physiologically relevant cell lines.IMPORTANCE The HIV-1 Env cytoplasmic tail (CT) is required for efficient Env incorporation into nascent particles and viral transmission in primary CD4⁺ T cells. The MT-4 T-cell line has been reported to support multiple rounds of infection of HIV-1 encoding a gp41 CT truncation. Uncovering the underlying mechanism of MT-4 T-cell line permissivity to gp41 CT truncation would provide key insights into the role of the gp41 CT in HIV-1 transmission. This study reveals that multiple factors contribute to the unique ability of a gp41 CT truncation mutant to spread in cultures of MT-4 cells. The lack of a requirement for the gp41 CT in MT-4 cells is associated with the combined effects of rapid HIV-1 protein production, high levels of cell-surface Env expression, and increased susceptibility to cell-to-cell transmission compared to nonpermissive cells.

Oregano Oil and Its Principal Component, Carvacrol, Inhibit HIV-1 Fusion into Target Cells

Oregano essential oil has long been known for its health-promoting benefits. Here, we report its activity against viral replication. Oregano oil was found to specifically inhibit lentiviruses, such as human and simian immunodeficiency viruses (HIV and SIV), irrespective of virus tropism, but not hepatitis C virus, adenovirus 5 (ADV5), Zika virus, and influenza (H1N1) virus. Oregano oil's most abundant components, carvacrol and its isomer, thymol, were shown to block virus-target cell fusion while not perturbing other stages of the virus life cycle. We detected changes in virus particle density, suggesting that cholesterol depletion from the HIV-1 envelope membrane reduces virus entry. Furthermore, infection was rescued by adding exogenous cholesterol. The evolution of viral resistance to carvacrol supported this mechanism of action with the identification of mutations in the viral gp41 fusion protein that counteracted cholesterol depletion. In addition, resistance to carvacrol emerged later than typically observed for other clinically used drugs, strengthening its antiviral potential. Structure-activity relationship studies revealed key motifs of carvacrol and thymol required for HIV neutralization and identified previously unknown active analogs. Carvacrol was also shown to additively cooperate with antiretroviral therapy. In sum, oregano oil and improved carvacrol and thymol analogs could be considered to supplement current HIV therapeutics.IMPORTANCE Oregano essential oil has multiple benefits in traditional medicine, cosmetics, and food industries. Carvacrol and its analog, thymol, are well-described components of oregano oil. Here, we show that these compounds inhibit HIV-target cell fusion independently of viral tropism. Our results suggest that carvacrol and thymol alter the cholesterol content of the viral membrane, blocking HIV-1 entry into the target cell. Resistance to carvacrol has selected for viruses with mutations in the viral envelope glycoprotein, gp41. This protein is known for its interaction with cholesterol present in membrane lipid rafts. Together, these results demonstrate the potential of therapies targeting the viral envelope membrane, and oregano oil is a safe supplement to antiretrovirals, potentially delaying disease progression and resistance development.

Structural basis of transmembrane coupling of the HIV-1 envelope glycoprotein

The prefusion conformation of HIV-1 envelope protein (Env) is recognized by most broadly neutralizing antibodies (bnAbs). Studies showed that alterations of its membrane-related components, including the transmembrane domain (TMD) and cytoplasmic tail (CT), can reshape the antigenic structure of the Env ectodomain. Using nuclear magnetic resonance (NMR) spectroscopy, we determine the structure of an Env segment encompassing the TMD and a large portion of the CT in bicelles. The structure reveals that the CT folds into amphipathic helices that wrap around the C-terminal end of the TMD, thereby forming a support baseplate for the rest of Env. NMR dynamics measurements provide evidences of dynamic coupling across the TMD between the ectodomain and CT. Pseudovirus-based neutralization assays suggest that CT-TMD interaction preferentially affects antigenic structure near the apex of the Env trimer. These results explain why the CT can modulate the Env antigenic properties and may facilitate HIV-1 Env-based vaccine design.

P2X1 Selective Antagonists Block HIV-1 Infection through Inhibition of Envelope Conformation-Dependent Fusion

Purinergic receptors are well-established modulators of inflammatory processes, primarily through detection of extracellular nucleotides that are released by dying or infected cells. Emerging literature has demonstrated that inhibition of these inflammatory receptors can block HIV-1 productive infection and HIV-1-associated inflammation. The specificity of receptor type and mechanism of interaction has not yet been determined. Here, we characterize the inhibitory activity of P2X1 receptor antagonists, NF279 and NF449, in cell lines, primary cells, and a variety of HIV-1 envelope (Env) clades. NF279 and NF449 blocked productive infection at the level of viral membrane fusion, with a range of inhibitory activities against different HIV-1 Env isolates. A mutant virus carrying a truncation deletion of the C-terminal tail of HIV-1 Env glycoprotein 41 (gp41) showed reduced sensitivity to P2X1 antagonists, indicating that the sensitivity of inhibition by these molecules may be modulated by Env conformation. In contrast, a P2X7 antagonist, A438079, had a limited effect on productive infection and fusion. NF279 and NF449 interfered with the ability of the gp120 variable regions 1 and 2 (V1V2)-targeted broadly neutralizing antibody PG9 to block productive infection, suggesting that these drugs may antagonize HIV-1 Env at gp120 V1V2 to block viral membrane fusion. Our observations indicate that P2X1 antagonism can inhibit HIV-1 replication at the level of viral membrane fusion through interaction with Env. Future studies will probe the nature of these compounds in inhibiting HIV-1 fusion and the development of small molecules to block HIV-1 entry via this mechanism.IMPORTANCE While effective treatment can lower the severe morbidity and mortality associated with HIV-1 infection, patients infected with HIV-1 suffer from significantly higher rates of noncommunicable comorbidities associated with chronic inflammation. Emerging literature suggests a key role for P2X1 receptors in mediating this chronic inflammation, but the mechanism is still unknown. Here, we demonstrate that HIV-1 infection is reduced by P2X1 receptor antagonism. This inhibition is mediated by interference with HIV-1 Env and can impact a variety of viral clades. These observations highlight the importance of P2X1 antagonists as potential novel therapeutics that could serve to block a variety of different viral clades with additional benefits for their anti-inflammatory properties.

Preclinical Optimization of gp120 Entry Antagonists as anti-HIV-1 Agents with Improved Cytotoxicity and ADME Properties through Rational Design, Synthesis, and Antiviral Evaluation

We previously reported a milestone in the optimization of NBD-11021, an HIV-1 gp120 antagonist, by developing a new and novel analogue, NBD-14189 (Ref1), which showed antiviral activity against HIV-1_HXB2, with a half maximal inhibitory concentration of 89 nM. However, cytotoxicity remained high, and the absorption, distribution, metabolism, and excretion (ADME) data showed relatively poor aqueous solubility. To optimize these properties, we replaced the phenyl ring in the compound with a pyridine ring and synthesized a set of 48 novel compounds. One of the new analogues, NBD-14270 (8), showed a marked improvement in cytotoxicity, with 3-fold and 58-fold improvements in selectivity index value compared with that of Ref1 and NBD-11021, respectively. Furthermore, the in vitro ADME data clearly showed improvements in aqueous solubility and other properties compared with those for Ref1. The data for 8 indicated that the pyridine scaffold is a good bioisostere for phenyl, allowing the further optimization of this molecule.

P2X Antagonists Inhibit HIV-1 Productive Infection and Inflammatory Cytokines Interleukin-10 (IL-10) and IL-1β in a Human Tonsil Explant Model

HIV-1 causes a persistent infection of the immune system that is associated with chronic comorbidities. The mechanisms that underlie this inflammation are poorly understood. Emerging literature has implicated proinflammatory purinergic receptors and downstream signaling mediators in HIV-1 infection. This study probed whether inhibitors of purinergic receptors would reduce HIV-1 infection and HIV-1-stimulated inflammation. An ex vivo human tonsil histoculture infection model was developed to support HIV-1 productive infection and stimulated the inflammatory cytokine interleukin-1 beta (IL-1β) and the immunosuppressive cytokine interleukin-10 (IL-10). This study tests whether inhibitors of purinergic receptors would reduce HIV-1 infection and HIV-1-stimulated inflammation. The purinergic P2X1 receptor antagonist NF449, the purinergic P2X7 receptor antagonist A438079, and azidothymidine (AZT) were tested in HIV-1-infected human tonsil explants to compare levels of inhibition of HIV-1 infection and HIV-stimulated inflammatory cytokine production. All drugs limited HIV-1 productive infection, but P2X-selective antagonists (NF449 and A438079) significantly lowered HIV-stimulated IL-10 and IL-1β. We further observed that P2X1- and P2X7-selective antagonists can act differentially as inhibitors of both HIV-1 infection and HIV-1-stimulated inflammation. Our findings highlight the differential effects of HIV-1 on inflammation in peripheral blood compared to those in lymphoid tissue. For the first time, we demonstrate that P2X-selective antagonists act differentially as inhibitors of both HIV-1 infection and HIV-1-stimulated inflammation. Drugs that block these pathways can have independent inhibitory activities against HIV-1 infection and HIV-induced inflammation.IMPORTANCE Patients who are chronically infected with HIV-1 experience sequelae related to chronic inflammation. The mechanisms of this inflammation have not been elucidated. Here, we describe a class of drugs that target the P2X proinflammatory signaling receptors in a human tonsil explant model. This model highlights differences in HIV-1 stimulation of lymphoid tissue inflammation and peripheral blood. These drugs serve to block both HIV-1 infection and production of IL-10 and IL-1β in lymphoid tissue, suggesting a novel approach to HIV-1 therapeutics in which both HIV-1 replication and inflammatory signaling are simultaneously targeted.

Variable infectivity and conserved engagement in cell-to-cell viral transfer by HIV-1 Env from Clade B transmitted founder clones

HIV-1 transmission is usually initiated by a single viral strain called transmitted/ founder (T/F) virus. In in vitro models, HIV-1 can efficiently spread via cell-free and virological synapse (VS)-mediated cell-to-cell infection. Both modes of infection require the viral glycoprotein Envelope (Env). The efficiency with which T/F Envs initiate VS and mediate cell-to-cell infection has not been well characterized. Here we tested a panel of isogenic HIV-1 molecular clones that carry different Clade B T/F Envs. We found that despite variable infectivity among different Env clones in the two modes of infection, T/F Envs generally mediated efficient VS formation and subsequent cell-to-cell transfer. In contrast, in vitro infectivity of the T/F Env clones was more variable and strongly correlated with intrinsic fusogenicity of various Envs. We speculate that the conservation of cell-to-cell transfer by T/F Env is indicative of a biologically important function of Env.

HIV-1 cell-to-cell transmission and broadly neutralizing antibodies

HIV-1 spreads through contacts between infected and target cells. Polarized viral budding at the contact site forms the virological synapse. Additional cellular processes, such as nanotubes, filopodia, virus accumulation in endocytic or phagocytic compartments promote efficient viral propagation. Cell-to-cell transmission allows immune evasion and likely contributes to HIV-1 spread in vivo. Anti-HIV-1 broadly neutralizing antibodies (bNAbs) defeat the majority of circulating viral strains by binding to the viral envelope glycoprotein (Env). Several bNAbs have entered clinical evaluation during the last years. It is thus important to understand their mechanism of action and to determine how they interact with infected cells. In experimental models, HIV-1 cell-to-cell transmission is sensitive to neutralization, but the effect of antibodies is often less marked than during cell-free infection. This may be due to differences in the conformation or accessibility of Env at the surface of virions and cells. In this review, we summarize the current knowledge on HIV-1 cell-to-cell transmission and discuss the role of bNAbs during this process.

Structure-based lead optimization to improve antiviral potency and ADMET properties of phenyl-1H-pyrrole-carboxamide entry inhibitors targeted to HIV-1 gp120

We are continuing our concerted effort to optimize our first lead entry antagonist, NBD-11021, which targets the Phe43 cavity of the HIV-1 envelope glycoprotein gp120, to improve antiviral potency and ADMET properties. In this report, we present a structure-based approach that helped us to generate working hypotheses to modify further a recently reported advanced lead entry antagonist, NBD-14107, which showed significant improvement in antiviral potency when tested in a single-cycle assay against a large panel of Env-pseudotyped viruses. We report here the synthesis of twenty-nine new compounds and evaluation of their antiviral activity in a single-cycle and multi-cycle assay to derive a comprehensive structure-activity relationship (SAR). We have selected three inhibitors with the high selectivity index for testing against a large panel of 55 Env-pseudotyped viruses representing a diverse set of clinical isolates of different subtypes. The antiviral activity of one of these potent inhibitors, 55 (NBD-14189), against some clinical isolates was as low as 63 nM. We determined the sensitivity of CD4-binding site mutated-pseudoviruses to these inhibitors to confirm that they target HIV-1 gp120. Furthermore, we assessed their ADMET properties and compared them to the clinical candidate attachment inhibitor, BMS-626529. The ADMET data indicate that some of these new inhibitors have comparable ADMET properties to BMS-626529 and can be optimized further to potential clinical candidates.

Neutralizing Antibody-Based Prevention of Cell-Associated HIV-1 Infection

Improved vaccine-mediated protection against HIV-1 requires a thorough understanding of the mode of HIV-1 transmission and how various immune responses control transmission. Cell-associated HIV-1 is infectious and contributes to HIV-1 transmission in humans. Non-human primate models of cell-associated SIV infection demonstrate that cell-associated SIV is more infectious than cell-free SIV. In a recently described chimeric simian–human immunodeficiency virus (SHIV) macaque model, it was demonstrated that an occult infection with cell-associated SHIV can be established that evades passive protection with a broadly neutralizing antibody (bnAb). Indeed, considerable in vitro data shows that bnAbs have less efficacy against cell-associated HIV-1 than cell-free HIV-1. Optimizing the protective capacity of immune responses such as bnAbs against cell-associated infections may be needed to maximize their protective efficacy.

Distinct functions for the membrane-proximal ectodomain region (MPER) of HIV-1 gp41 in cell-free and cell-cell viral transmission and cell-cell fusion

HIV-1 is spread by cell-free virions and by cell-cell viral transfer. We asked whether the structure and function of a broad neutralizing antibody (bNAb) epitope, the membrane-proximal ectodomain region (MPER) of the viral gp41 transmembrane glycoprotein, differ in cell-free and cell-cell-transmitted viruses and whether this difference could be related to Ab neutralization sensitivity. Whereas cell-free viruses bearing W666A and I675A substitutions in the MPER lacked infectivity, cell-associated mutant viruses were able to initiate robust spreading infection. Infectivity was restored to cell-free viruses by additional substitutions in the cytoplasmic tail (CT) of gp41 known to disrupt interactions with the viral matrix protein. We observed contrasting effects on cell-free virus infectivity when W666A was introduced to two transmitted/founder isolates, but both mutants could still mediate cell-cell spread. Domain swapping indicated that the disparate W666A phenotypes of the cell-free transmitted/founder viruses are controlled by sequences in variable regions 1, 2, and 4 of gp120. The sequential passaging of an MPER mutant (W672A) in peripheral blood mononuclear cells enabled selection of viral revertants with loss-of-glycan suppressor mutations in variable region 1, suggesting a functional interaction between variable region 1 and the MPER. An MPER-directed bNAb neutralized cell-free virus but not cell-cell viral spread. Our results suggest that the MPER of cell-cell-transmitted virions has a malleable structure that tolerates mutagenic disruption but is not accessible to bNAbs. In cell-free virions, interactions mediated by the CT impose an alternative MPER structure that is less tolerant of mutagenic alteration and is efficiently targeted by bNAbs.

Reassessment of the capacity of the HIV-1 Env cytoplasmic domain to trigger NF-κB activation

The cytoplasmic domain of lentiviral Envelopes (EnvCD) ensures Env incorporation into nascent virions and regulates Env trafficking to and from the plasma membrane. It has also been reported to promote transcription from the viral LTR both directly and indirectly. Noticeably, the HIV-1 and SIV_mac239 EnvCDs were described to trigger nuclear translocation of NF-κB (Postler, Cell Host Microbes 2012). Given the paramount importance of identifying viral and host factors regulating HIV transcription, cellular signaling pathways and latency, and given that viral replication capacity is dependent on Env, we asked whether HIV EnvCDs from different HIV-1 subtypes differently modulated NF-κB. To that aim, we evaluated the ability of primary HIV-1 Envs from subtypes B and C to activate the NF-κB pathway. Primary subtype B and C Envs all failed to activate the NF-κB pathway. In contrast, when the EnvCD of HIV-1 Envs was fused to the the CD8-α chain, it induced ~ 10-fold increase in NF-κB induction, and this increase was much stronger with a truncated form of the HIV EnvCD lacking the 76 C-terminal residues and containing the proposed TAK-1 binding domain. Our results indicate that the HIV-1 EnvCD is unlikely to trigger the NF-κB pathway in its native trimeric form.

Phenotypic deficits in the HIV-1 envelope are associated with the maturation of a V2-directed broadly neutralizing antibody lineage

Broadly neutralizing antibodies (bnAbs) to HIV-1 can evolve after years of an iterative process of virus escape and antibody adaptation that HIV-1 vaccine design seeks to mimic. To enable this, properties that render HIV-1 envelopes (Env) capable of eliciting bnAb responses need to be defined. Here, we followed the evolution of the V2 apex directed bnAb lineage VRC26 in the HIV-1 subtype C superinfected donor CAP256 to investigate the phenotypic changes of the virus populations circulating before and during the early phases of bnAb induction. Longitudinal viruses that evolved from the VRC26-resistant primary infecting (PI) virus, the VRC26-sensitive superinfecting (SU) virus and ensuing PI-SU recombinants revealed substantial phenotypic changes in Env, with a switch in Env properties coinciding with early resistance to VRC26. Decreased sensitivity of SU-like viruses to VRC26 was linked with reduced infectivity, altered entry kinetics and lower sensitivity to neutralization after CD4 attachment. VRC26 maintained neutralization activity against cell-associated CAP256 virus, indicating that escape through the cell-cell transmission route is not a dominant escape pathway. Reduced fitness of the early escape variants and sustained sensitivity in cell-cell transmission are both features that limit virus replication, thereby impeding rapid escape. This supports a scenario where VRC26 allowed only partial viral escape for a prolonged period, possibly increasing the time window for bnAb maturation. Collectively, our data highlight the phenotypic plasticity of the HIV-1 Env in evading bnAb pressure and the need to consider phenotypic traits when selecting and designing Env immunogens. Combinations of Env variants with differential phenotypic patterns and bnAb sensitivity, as we describe here for CAP256, may maximize the potential for inducing bnAb responses by vaccination.

HIV-1 infected individuals rarely develop broadly neutralizing antibodies (bnAbs) that inhibit diverse HIV-1 subtypes. As activity against the majority of HIV-1 isolates is necessary for effective immunization against HIV-1, current vaccine development seeks to generate regimens that evoke bnAb responses. Delineating why bnAbs develop in certain cases of HIV-1 infection is thus of pivotal importance. In all infected individuals, HIV-1 inevitably escapes neutralizing antibody pressure and even the most potent bnAbs cannot clear the infection from the patient where they emerged. Yet, exactly this continuous interplay between virus escape and antibody maturation is believed to be crucial in the evolution of bnAbs, with virus escape variants containing modified HIV-1 envelope (Env) proteins, the target of neutralizing antibodies that in some cases can direct the immune response towards breadth. Identifying features of naturally occurring Env proteins that are involved in evoking bnAb responses is thus of high interest. Here, we analyzed Env features of virus isolates from donor CAP256 who developed the potent V2 apex bnAb VRC26, one of the current lead bnAbs for HIV-1 therapy and vaccine development. We show that the viral escape variants that appeared soon after the onset of the VRC26 response had highly altered Env protein properties that, in addition to reducing sensitivity to VRC26, affected their capacity to infect and altered entry dynamics. This highlights constrained viral escape pathways, but also features of VRC26 that may have prevented rapid escape. We postulate that this may have resulted in a prolonged circulation of partially VRC26 sensitive viruses, hence allowing the bnAb response to mature.

Non-Neutralizing Antibodies Directed against HIV and Their Functions

B cells produce a plethora of anti-HIV antibodies (Abs) but only few of them exhibit neutralizing activity. This was long considered a profound limitation for the enforcement of humoral immune responses against HIV-1 infection, especially since these neutralizing Abs (nAbs) are extremely difficult to induce. However, increasing evidence shows that additional non-neutralizing Abs play a significant role in decreasing the viral load, leading to partial and sometimes even total protection. Mechanisms suspected to participate in protection are numerous. They involve the Fc domain of Abs as well as their Fab part, and consequently the induced Ab isotype will be determinant for their functions, as well as the quantity and quality of the Fc-receptors (FcRs) expressed on immune cells. Fc-mediated inhibitory functions, such as Ab-dependent cellular cytotoxicity, antibody-dependent cellular phagocytosis, aggregation, and even immune activation have been proposed. However, as for nAbs, the non-neutralizing activities are limited to a subset of anti-HIV Abs. An improved in-depth characterization of the Abs displaying these functional responses is required for the development of new vaccination strategies, which aim to selectively trigger the B cells able to induce the right functional Ab combinations both at the right place and at the right time. This review summarizes our current knowledge on non-neutralizing functional inhibitory Abs and discusses the potential benefit of inducing them via vaccination. We also provide new insight into the roles of the FcγR-mediated Ab therapeutics in clinical trials for HIV diseases.

Dense Array of Spikes on HIV-1 Virion Particles

HIV-1 is rare among viruses for having a low number of envelope glycoprotein (Env) spikes per virion, i.e., ∼7 to 14. This exceptional feature has been associated with avoidance of humoral immunity, i.e., B cell activation and antibody neutralization. Virus-like particles (VLPs) with increased density of Env are being pursued for vaccine development; however, these typically require protein engineering that alters Env structure. Here, we used instead a strategy that targets the producer cell. We employed fluorescence-activated cell sorting (FACS) to sort for cells that are recognized by trimer cross-reactive broadly neutralizing antibody (bnAb) and not by nonneutralizing antibodies. Following multiple iterations of FACS, cells and progeny virions were shown to display higher levels of antigenically correct Env in a manner that correlated between cells and cognate virions (P = 0.027). High-Env VLPs, or hVLPs, were shown to be monodisperse and to display more than a 10-fold increase in spikes per particle by electron microscopy (average, 127 spikes; range, 90 to 214 spikes). Sequencing revealed a partial truncation in the C-terminal tail of Env that had emerged in the sort; however, iterative rounds of "cell factory" selection were required for the high-Env phenotype. hVLPs showed greater infectivity than standard pseudovirions but largely similar neutralization sensitivity. Importantly, hVLPs also showed superior activation of Env-specific B cells. Hence, high-Env HIV-1 virions, obtained through selection of producer cells, represent an adaptable platform for vaccine design and should aid in the study of native Env.IMPORTANCE The paucity of spikes on HIV is a unique feature that has been associated with evasion of the immune system, while increasing spike density has been a goal of vaccine design. Increasing the density of Env by modifying it in various ways has met with limited success. Here, we focused instead on the producer cell. Cells that stably express HIV spikes were screened on the basis of high binding by bnAbs and low binding by nonneutralizing antibodies. Levels of spikes on cells correlated well with those on progeny virions. Importantly, high-Env virus-like particles (hVLPs) were produced with a manifest array of well-defined spikes, and these were shown to be superior in activating desirable B cells. Our study describes HIV particles that are densely coated with functional spikes, which should facilitate the study of HIV spikes and their development as immunogens.

The pentameric complex drives immunologically covert cell-cell transmission of wild-type human cytomegalovirus

Human cytomegalovirus (HCMV) strains that have been passaged in vitro rapidly acquire mutations that impact viral growth. These laboratory-adapted strains of HCMV generally exhibit restricted tropism, produce high levels of cell-free virus, and develop susceptibility to natural killer cells. To permit experimentation with a virus that retained a clinically relevant phenotype, we reconstructed a wild-type (WT) HCMV genome using bacterial artificial chromosome technology. Like clinical virus, this genome proved to be unstable in cell culture; however, propagation of intact virus was achieved by placing the RL13 and UL128 genes under conditional expression. In this study, we show that WT-HCMV produces extremely low titers of cell-free virus but can efficiently infect fibroblasts, epithelial, monocyte-derived dendritic, and Langerhans cells via direct cell-cell transmission. This process of cell-cell transfer required the UL128 locus, but not the RL13 gene, and was significantly less vulnerable to the disruptive effects of IFN, cellular restriction factors, and neutralizing antibodies compared with cell-free entry. Resistance to neutralizing antibodies was dependent on high-level expression of the pentameric gH/gL/gpUL128-131A complex, a feature of WT but not passaged strains of HCMV.

New Connections: Cell-to-Cell HIV-1 Transmission, Resistance to Broadly Neutralizing Antibodies, and an Envelope Sorting Motif

HIV-1 infection from cell-to-cell may provide an efficient mode of viral spread in vivo and could therefore present a significant challenge for preventative or therapeutic strategies based on broadly neutralizing antibodies. Indeed, Li et al. (H. Li, C. Zony, P. Chen, and B. K. Chen, J. Virol. 91:e02425-16, 2017, https://doi.org/10.1128/JVI.02425-16) showed that the potency and magnitude of multiple HIV-1 broadly neutralizing antibody classes are decreased during cell-to-cell infection in a context-dependent manner. A functional motif in gp41 appears to contribute to this differential susceptibility by modulating exposure of neutralization epitopes.

Reduced Potency and Incomplete Neutralization of Broadly Neutralizing Antibodies against Cell-to-Cell Transmission of HIV-1 with Transmitted Founder Envs

Broadly neutralizing antibodies (bNAbs) have been isolated from HIV-1 patients and can potently block infection of a wide spectrum of HIV-1 subtypes. These antibodies define common epitopes shared by many viral isolates. While bNAbs potently antagonize infection with cell-free virus, inhibition of HIV-1 transmission from infected to uninfected CD4⁺ T cells through virological synapses (VS) has been found to require greater amounts of antibody. In this study, we examined two well-studied molecular clones and two transmitted/founder (T/F) clones for their sensitivities to a panel of bNAbs in cell-free and cell-to-cell infection assays. We observed resistance of cell-to-cell transmission to antibody neutralization that was reflected not only by reductions of antibody potency but also by decreases in maximum neutralization capacity relative to the levels seen with cell-free infections. BNAbs targeting different epitopes exhibited incomplete neutralization against cell-associated virus with T/F Envs, which was not observed with the cell-free form of the same virus. We further identified the membrane-proximal internal tyrosine-based sorting motif as a determinant that can affect the incomplete neutralization of these T/F clones in cell-to-cell infection. These findings indicate that the signal that affects surface expression and/or internalization of Env from the plasma membrane can modulate the presentation of neutralizing epitopes on infected cells. These results highlight that a fraction of virus can escape from high concentrations of antibody through cell-to-cell infection while remaining sensitive to neutralization in cell-free infection. The ability to fully inhibit cell-to-cell transmission may represent an important consideration in the development of antibodies for treatment or prophylaxis.IMPORTANCE In recent years, isolation of new-generation HIV-1 bNAbs has invigorated HIV vaccine research. These bNAbs display remarkable potency and breadth of coverage against cell-free virus; however, they exhibit a diminished ability to block HIV-1 cell-to-cell transmission. The mechanism(s) by which HIV-1 resists neutralization when transmitting through VS remains uncertain. We examined a panel of bNAbs for their ability to neutralize HIV-1 T/F viruses in cell-to-cell infection assays. We found that some antibodies exhibit not only reduced potency but also decreased maximum neutralization capacity or in vitro efficacy against cell-to-cell infection of HIV-1 with T/F Envs compared to cell-free infection of the same virus. We further identified the membrane-proximal internal tyrosine-based sorting motif YXXL as a determinant that can affect the incomplete neutralization phenotype of these T/F clones. When the maximum neutralization capacity falls short of 100%, this can have a major impact on the ability of antibodies to halt viral replication.

The Glycosylphosphatidylinositol-Anchored Variable Region of Llama Heavy Chain-Only Antibody JM4 Efficiently Blocks both Cell-Free and T Cell-T Cell Transmission of Human Immunodeficiency Virus Type 1

The variable regions (VHHs) of two heavy chain-only antibodies, JM2 and JM4, from llamas that have been immunized with a trimeric gp140 bound to a CD4 mimic have been recently isolated (here referred to as VHH JM2 and VHH JM4, respectively). JM2 binds the CD4-binding site of gp120 and neutralizes HIV-1 strains from subtypes B, C, and G. JM4 binds gp120 and neutralizes HIV-1 strains from subtypes A, B, C, A/E, and G in a CD4-dependent manner. In the present study, we constructed glycosylphosphatidylinositol (GPI)-anchored VHH JM2 and JM4 along with an E4 control and transduced them into human CD4⁺ cell lines and primary CD4 T cells. We report that by genetically linking the VHHs with a GPI attachment signal, VHHs are targeted to the lipid rafts of the plasma membranes. Expression of GPI-VHH JM4, but not GPI-VHH E4 and JM2, on the surface of transduced TZM.bl cells potently neutralizes multiple subtypes of HIV-1 isolates, including tier 2 or 3 strains, transmitted founders, quasispecies, and soluble single domain antibody (sdAb) JM4-resistant viruses. Moreover, transduction of CEMss-CCR5 cells with GPI-VHH JM4, but not with GPI-VHH E4, confers resistance to both cell-free and T cell-T cell transmission of HIV-1 and HIV-1 envelope-mediated fusion. Finally, GPI-VHH JM4-transduced human primary CD4 T cells efficiently resist both cell-free and T cell-T cell transmission of HIV-1. Thus, we conclude that VHH JM4, when targeted to the lipid rafts of the plasma membrane, efficiently neutralizes HIV-1 infection via both cell-free and T cell-T cell transmission. Our findings should have important implications for GPI-anchored antibody-based therapy against HIV-1.

IMPORTANCE

Lipid rafts are specialized dynamic microdomains of the plasma membrane and have been shown to be gateways for HIV-1 budding as well as entry into T cells and macrophages. In nature, many glycosylphosphatidylinositol (GPI)-anchored proteins localize in the lipid rafts. In the present study, we developed GPI-anchored variable regions (VHHs) of two heavy chain-only antibodies, JM2 and JM4, from immunized llamas. We show that by genetically linking the VHHs with a GPI attachment signal, VHHs are targeted to the lipid rafts of the plasma membranes. GPI-VHH JM4, but not GPI-VHH JM2, in transduced CD4⁺ cell lines and human primary CD4 T cells not only efficiently blocks diverse HIV-1 strains, including tier 2 or 3 strains, transmitted founders, quasispecies, and soluble sdAb JM4-resistant strains, but also efficiently interferes T cell-T cell transmissions of HIV-1 and HIV-1 envelope-mediated fusion. Our findings should have important implications in GPI-anchored antibody-based therapy against HIV-1.

Broadly neutralizing antibodies: An approach to control HIV-1 infection

Although available antiretroviral therapy (ART) has changed human immunodeficiency virus (HIV)-1 infection to a non-fatal chronic disease, the economic burden of lifelong therapy, severe adverse ART effects, daily ART adherence, and emergence of ART-resistant HIV-1 mutants require prospecting for alternative therapeutic modalities. Indeed, a growing body of evidence suggests that broadly neutralizing anti-HIV-1 antibodies (BNAbs) may offer one such feasible alternative. To evaluate their therapeutic potential in established HIV-1 infection, we sought to address recent advances in pre-clinical and clinical investigations in this area of HIV-1 research. In addition, we addressed the obstacles that may impede the success of such immunotherapeutic approach, suggested strategic solutions, and briefly compared this approach with the currently used ART to open new insights for potential future passive immunotherapy for HIV-1 infection.

The Envelope Cytoplasmic Tail of HIV-1 Subtype C Contributes to Poor Replication Capacity through Low Viral Infectivity and Cell-to-Cell Transmission

The cytoplasmic tail (gp41CT) of the HIV-1 envelope (Env) mediates Env incorporation into virions and regulates Env intracellular trafficking. Little is known about the functional impact of variability in this domain. To address this issue, we compared the replication of recombinant virus pairs carrying the full Env (Env viruses) or the Env ectodomain fused to the gp41CT of NL4.3 (EnvEC viruses) (12 subtype C and 10 subtype B pairs) in primary CD4+ T-cells and monocyte-derived-macrophages (MDMs). In CD4+ T-cells, replication was as follows: B-EnvEC = B-Env>C-EnvEC>C-Env, indicating that the gp41CT of subtype C contributes to the low replicative capacity of this subtype. In MDMs, in contrast, replication capacity was comparable for all viruses regardless of subtype and of gp41CT. In CD4+ T-cells, viral entry, viral release and viral gene expression were similar. However, infectivity of free virions and cell-to-cell transmission of C-Env viruses released by CD4+ T-cells was lower, suggestive of lower Env incorporation into virions. Subtype C matrix only minimally rescued viral replication and failed to restore infectivity of free viruses and cell-to-cell transmission. Taken together, these results show that polymorphisms in the gp41CT contribute to viral replication capacity and suggest that the number of Env spikes per virion may vary across subtypes. These findings should be taken into consideration in the design of vaccines.

In Vivo HIV-1 Cell-to-Cell Transmission Promotes Multicopy Micro-compartmentalized Infection

HIV-1 infection is enhanced by adhesive structures that form between infected and uninfected T cells called virological synapses (VSs). This mode of transmission results in the frequent co-transmission of multiple copies of HIV-1 across the VS, which can reduce sensitivity to antiretroviral drugs. Studying HIV-1 infection of humanized mice, we measured the frequency of co-transmission and the spatiotemporal organization of infected cells as indicators of cell-to-cell transmission in vivo. When inoculating mice with cells co-infected with two viral genotypes, we observed high levels of co-transmission to target cells. Additionally, micro-anatomical clustering of viral genotypes within lymphoid tissue indicates that viral spread is driven by local processes and not a diffuse viral cloud. Intravital splenic imaging reveals that anchored HIV-infected cells induce arrest of interacting, uninfected CD4(+) T cells to form Env-dependent cell-cell conjugates. These findings suggest that HIV-1 spread between immune cells can be anatomically localized into infectious clusters.

Cell-to-Cell Spread of HIV and Viral Pathogenesis

Human immunodeficiency virus type 1 (HIV-1) gives rise to a chronic infection that progressively depletes CD4(+) T lymphocytes. CD4(+) T lymphocytes play a central coordinating role in adaptive cellular and humoral immune responses, and to do so they migrate and interact within lymphoid compartments and at effector sites to mount immune responses. While cell-free virus serves as an excellent prognostic indicator for patient survival, interactions of infected T cells or virus-scavenging immune cells with uninfected T cells can greatly enhance viral spread. HIV can induce interactions between infected and uninfected T cells that are triggered by cell surface expression of viral Env, which serves as a cell adhesion molecule that interacts with CD4 on the target cell, before it acts as the viral membrane fusion protein. These interactions are called virological synapses and promote replication in the face of selective pressure of humoral immune responses and antiretroviral therapy. Other infection-enhancing cell-cell interactions occur between virus-concentrating antigen-presenting cells and recipient T cells, called infectious synapses. The exact roles that these cell-cell interactions play in each stage of infection, from viral acquisition, systemic dissemination, to chronic persistence are still being determined. Infection-promoting immune cell interactions are likely to contribute to viral persistence and enhance the ability of HIV-1 to evade adaptive immune responses.

A high throughput Cre-lox activated viral membrane fusion assay identifies pharmacological inhibitors of HIV entry

Enveloped virus entry occurs when viral and cellular membranes fuse releasing particle contents into the target cell. Human immunodeficiency virus (HIV) entry occurs by cell-free virus or virus transferred between infected and uninfected cells through structures called virological synapses. We developed a high-throughput cell-based assay to identify small molecule inhibitors of cell-free or virological synapse-mediated entry. An HIV clone carrying Cre recombinase as a Gag-internal gene fusion releases active Cre into cells upon viral entry activating a recombinatorial gene switch changing dsRed to GFP-expression. A screen of a 1998 known-biological profile small molecule library identified pharmacological HIV entry inhibitors that block both cell-free and cell-to-cell infection. Many top hits were noted as HIV inhibitors in prior studies, but not previously recognized as entry antagonists. Modest therapeutic indices for simvastatin and nigericin were observed in confirmatory HIV infection assays. This robust assay is adaptable to study HIV and heterologous viral pseudotypes.

Measuring T Cell-to-T Cell HIV-1 Transfer, Viral Fusion, and Infection Using Flow Cytometry

Direct T cell-to-T cell HIV-1 infection is a distinct mode of HIV-1 infection that requires physical contact between an HIV-1-infected "donor" cell and an uninfected, CD4-expressing "target" cell. In vitro studies indicate that HIV-1 cell-to-cell infection is much more efficient than infection by cell-free viral particles; however, the exact mechanisms of the enhanced efficiency of this infection pathway are still unclear. Several assays have been developed to study the mechanism of direct cell-to-cell HIV-1 transmission and to assess sensitivity to neutralizing antibodies and pharmacologic inhibitors. These assays are based on the coculture of donor and target cells. Here, we describe methods that utilize flow cytometry, which can discriminate donor and target cells and can assess different stages of entry and infection following cell-to-cell contact. HIV Gag-iGFP, a clone that makes fluorescent virus particles, can be used to measure cell-to-cell transfer of virus particles. HIV NL-GI, a clone that expresses GFP as an early gene, facilitates the measure of productive infection after cell-to-cell contact. Lastly, a variation of the β-lactamase (BlaM)-Vpr fusion assay can be used to measure the viral membrane fusion process after coculture of donor and target cells in a manner that is independent of cell-cell fusion. These assays can be performed in the presence of neutralizing antibodies/inhibitors to determine the 50 % inhibitory concentration (IC50) required to block infection specifically in the target cells.

HIV-1 resistance to neutralizing antibodies: Determination of antibody concentrations leading to escape mutant evolution

Broadly neutralizing antibodies against human immunodeficiency virus type 1 (HIV-1) are considered vital components of novel therapeutics and blueprints for vaccine research. Yet escape to even the most potent of these antibodies is imminent in natural infection. Measures to define antibody efficacy and prevent mutant selection are thus urgently needed. Here, we derive a mathematical framework to predict the concentration ranges for which antibody escape variants can outcompete their viral ancestors, referred to as mutant selection window (MSW). When determining the MSW, we focus on the differential efficacy of neutralizing antibodies against HIV-1 in two canonical infection routes, free-virus infection and cell-cell transmission. The latter has proven highly effective in vitro suggesting its importance for both in vivo spread as well as for escaping targeted intervention strategies. We observed a range of MSW patterns that highlight the potential of mutants to arise in both transmission pathways and over wide concentration ranges. Most importantly, we found that only when the arising mutant has both, residual sensitivity to the neutralizing antibody and reduced infectivity compared to the parental virus, antibody dosing outside of the MSW to restrict mutant selection is possible. Emergence of mutants that provide complete escape and have no considerable fitness loss cannot be prevented by adjusting antibody doses. The latter may in part explain the ubiquitous resistance to neutralizing antibodies observed in natural infection and antibody treatment. Based on our findings, combinations of antibodies targeting different epitopes should be favored for antibody-based interventions as this may render complete resistance less likely to occur and also increase chances that multiple escapes result in severe fitness loss of the virus making longer-term antibody treatment more feasible.

Impact of HIV-1 infection pathways on susceptibility to antiviral drugs and on virus spread

The infection routes of HIV-1 can affect several viral properties, including dissemination, pathogenesis, and immune evasion. In this study, we evaluated the inhibitory activity of a wide variety of anti-HIV drugs, focusing on the impact that different infection pathways have on their efficacy. Compared to cell-free infection, inhibitory activities were reduced in cell-to-cell productive transmission for all drugs tested. We detected weak reporter-expressing target cells after cell-to-cell transmission in the presence of integrase strand transfer inhibitors (INSTIs). Further analysis revealed that this expression was mainly due to unintegrated circular HIV (cHIV) DNAs, consisting of 1-LTR and 2-LTR circles. When in vitro-constructed cHIV DNAs were introduced into cells, the production of infectious and intercellular transmittable virions was observed, suggesting that cHIV DNA could be a source of infectious virus. These results highlight some advantages of the cell-to-cell infection mode for viral expansion, particularly in the presence of anti-retroviral drugs.

Capacity of Broadly Neutralizing Antibodies to Inhibit HIV-1 Cell-Cell Transmission Is Strain- and Epitope-Dependent

An increasing number of broadly neutralizing antibodies (bnAbs) are considered leads for HIV-1 vaccine development and novel therapeutics. Here, we systematically explored the capacity of bnAbs to neutralize HIV-1 prior to and post-CD4 engagement and to block HIV-1 cell-cell transmission. Cell-cell spread is known to promote a highly efficient infection with HIV-1 which can inflict dramatic losses in neutralization potency compared to free virus infection. Selection of bnAbs that are capable of suppressing HIV irrespective of the transmission mode therefore needs to be considered to ascertain their in vivo activity in therapeutic use and vaccines. Employing assay systems that allow for unambiguous discrimination between free virus and cell-cell transmission to T cells, we probed a panel of 16 bnAbs for their activity against 11 viruses from subtypes A, B and C during both transmission modes. Over a wide range of bnAb-virus combinations tested, inhibitory activity against HIV-1 cell-cell transmission was strongly decreased compared to free virus transmission. Activity loss varied considerably between virus strains and was inversely associated with neutralization of free virus spread for V1V2- and V3-directed bnAbs. In rare bnAb-virus combinations, inhibition for both transmission modes was comparable but no bnAb potently blocked cell-cell transmission across all probed virus strains. Mathematical analysis indicated an increased probability of bnAb resistance mutations to arise in cell-cell rather than free virus spread, further highlighting the need to block this pathway. Importantly, the capacity to efficiently neutralize prior to CD4 engagement correlated with the inhibition efficacy against free virus but not cell-cell transmitted virus. Pre-CD4 attachment activity proved strongest amongst CD4bs bnAbs and varied substantially for V3 and V1V2 loop bnAbs in a strain-dependent manner. In summary, bnAb activity against divergent viruses varied depending on the transmission mode and differed depending on the window of action during the entry process, underscoring that powerful combinations of bnAbs are needed for in vivo application.

When selecting broadly neutralizing antibodies (bnAbs) for clinical application, potency and breadth against free viruses are vital, but additional features may be needed to ensure in vivo efficacy. Considering that HIV-1 can utilize free virus and cell-cell transmission to infect, the efficacy of neutralizing antibodies in vivo may depend on their ability to block both pathways. While breadth and potency of bnAbs against free viruses have been intensely studied, their precise activity during cell-cell spread remains uncertain. Our analysis of the cell-cell neutralization capacity of a large selection of bnAbs against a spectrum of HIV-1 strains revealed that while bnAbs showed an overall decreased activity during cell-cell transmission, losses varied substantially depending on bnAb and virus strain probed. Although bnAbs occasionally retained activity during cell-cell transmission for individual viruses, this ability was rare and generally not associated with a high potency against free virus spread. Notably, neutralization of free virus but not cell-cell transmission was linked with the activity of bnAbs to inhibit prior to CD4 engagement, highlighting the functional differences of the processes. Since no single bnAb combines the entire range of mechanistic features anticipated to support in vivo efficacy, our study adds further evidence that combinations of bnAbs need to be considered for human application.

HIV-1 Cell-Free and Cell-to-Cell Infections Are Differentially Regulated by Distinct Determinants in the Env gp41 Cytoplasmic Tail

The HIV-1 envelope (Env) glycoprotein mediates viral entry during both cell-free and cell-to-cell infection of CD4(+) T cells. The highly conserved long cytoplasmic tail (CT) of Env is required in a cell type-dependent manner for optimal infectivity of cell-free virus. To probe the role of the CT in cell-to-cell infection, we tested a panel of mutations in the CT region that maintain or attenuate cell-free infection to investigate whether the functions of the CT are conserved during cell-free and cell-to-cell infection. The mutations tested included truncations of structural motifs in the gp41 CT and two point mutations in lentiviral lytic peptide 3 (LLP-3) previously described as disrupting the infectivity of cell-free virus. We found that small truncations of 28 to 43 amino acids (aa) or two LLP-3 point mutations, YW_SL and LL_RQ, severely impaired single-round cell-free infectivity 10-fold or more relative to wild-type full-length CT. These mutants showed a modest 2-fold reduction in cell-to-cell infection assays. Conversely, large truncations of 93 to 124 aa severely impaired cell-to-cell infectivity 20-fold or more while resulting in a 50% increase in infectivity of cell-free viral particles when produced in 293T cells. Intermediate truncations of 46 to 90 aa showed profound impairment of both modes of infection. Our results show that the abilities of Env to support cell-free and cell-to-cell infection are genetically distinct. These differences are cell type dependent for large-CT-truncation mutants. Additionally, point mutants in LLP-3 can maintain multiround propagation from cell-to-cell in primary CD4(+) T cells.

IMPORTANCE

The functions of HIV Env gp41 CT remain poorly understood despite being widely studied in the context of cell-free infection. We have identified domains of the gp41 CT responsible for striking selective deficiencies in either cell-free or cell-to-cell infectivity. These differences may reflect a different intrinsic regulatory influence of the CT on cell-associated versus particle-associated Env or differential interaction with host or viral proteins. Our findings provide novel insight into the key regulatory potential of the gp41 CT in cell-free and cell-to-cell HIV-1 infection, particularly for short-truncation mutants of ≤43 amino acids or mutants with point mutations in the LLP-3 helical domain of the CT, which are able to propagate via cell-to-cell infection in the absence of infectious cell-free virus production. These mutants may also serve as tools to further define the contributions of cell-free and cell-to-cell infection in vitro and in vivo.

Vpr Promotes Macrophage-Dependent HIV-1 Infection of CD4+ T Lymphocytes

Vpr is a conserved primate lentiviral protein that promotes infection of T lymphocytes in vivo by an unknown mechanism. Here we demonstrate that Vpr and its cellular co-factor, DCAF1, are necessary for efficient cell-to-cell spread of HIV-1 from macrophages to CD4+ T lymphocytes when there is inadequate cell-free virus to support direct T lymphocyte infection. Remarkably, Vpr functioned to counteract a macrophage-specific intrinsic antiviral pathway that targeted Env-containing virions to LAMP1+ lysosomal compartments. This restriction of Env also impaired virological synapses formed through interactions between HIV-1 Env on infected macrophages and CD4 on T lymphocytes. Treatment of infected macrophages with exogenous interferon-alpha induced virion degradation and blocked synapse formation, overcoming the effects of Vpr. These results provide a mechanism that helps explain the in vivo requirement for Vpr and suggests that a macrophage-dependent stage of HIV-1 infection drives the evolutionary conservation of Vpr.

HIV cell-to-cell transmission: effects on pathogenesis and antiretroviral therapy

HIV spreads more efficiently in vitro when infected cells directly contact uninfected cells to form virological synapses. A hallmark of virological synapses is that viruses can be transmitted at a higher multiplicity of infection (MOI) that, in vitro, results in a higher number of proviruses. Whether HIV also spreads by cell-cell contact in vivo is a matter of debate. Here we discuss recent data that suggest that contact-mediated transmission largely manifests itself in vivo as CD4+ T cell depletion. The assault of a cell by a large number of incoming particles is likely to be efficiently sensed by the innate cellular surveillance to trigger cell death. The large number of particles transferred across virological synapses has also been implicated in reduced efficacy of antiretroviral therapies. Thus, antiretroviral therapies must remain effective against the high MOI observed during cell-to-cell transmission to inhibit both viral replication and the pathogenesis associated with HIV infection.

The membrane proximal external regions of gp41 from HIV-1 strains HXB2 and JRFL have different sensitivities to alanine mutation

The transmembrane subunit (gp41) of the HIV envelope protein complex (Env) mediates the viral fusion step of HIV entry. The membrane proximal external region (MPER), one of the functional domains of gp41, has been the focus of a great deal of research because it is a target for neutralizing antibodies. In this study, we examined 23 amino acid residues in the MPER (660-683) in both a CXCR4 coreceptor-utilizing strain (HXB2) and a CCR5-utilizing strain (JRFL) by alanine scanning mutagenesis. Despite the high degree of gp41 sequence conservation, the effects of alanine mutation in the MPER were different between the two strains. Most mutations in HXB2 had fusogenicity and protein expression levels not less than 50% of that of the wild type in the case of cell-cell fusion. However, ∼30% of the mutants in HXB2 showed a severe defect in fusogenicity in viral entry. Mutations in the MPER of strain JRFL had more dramatic effects than that in HXB2 in cell-cell fusion and viral entry. The fact that there are large differences in the effects of mutation between two strains suggests the potential for the interaction of the MPER with nonconserved sequences such as the fusion peptide and/or other NHR domains as well as potential long-range structural effects on the conformational changes that occur with the Env complex during membrane fusion.

The cytoplasmic tail of retroviral envelope glycoproteins

Retroviruses comprise a large, diverse group that infects a broad range of host organisms. Pathogenicity varies widely; the human immunodeficiency virus is the causative agent of acquired immunodeficiency syndrome, one of the world's leading infectious causes of death, while many nonhuman retroviruses cause cancer in the host. Retroviruses have been studied intensively, and great strides have been made in understanding aspects of retroviral biology. While the principal functions of the viral structural proteins are well understood, there remain many incompletely characterized domains. One of these is the cytoplasmic tail (CT) of the envelope glycoprotein. Several functions of the CT are highly conserved, whereas other properties are unique to a specific retrovirus. For example, the lentiviruses encode envelope glycoproteins with particularly large cytoplasmic domains. The functions of the long lentiviral envelope CT are still being deciphered. The reported functions of retroviral envelope CTs are discussed in this chapter.

Cell-to-cell transfer of HIV infection: implications for HIV viral persistence

A major research priority for HIV eradication is the elucidation of the events involved in HIV reservoir establishment and persistence. Cell-to-cell transmission of HIV represents an important area of study as it allows for the infection of cell types which are not easily infected by HIV, leading to the establishment of long-lived viral reservoirs. This phenomenon enables HIV to escape elimination by the immune system. This process may also enable HIV to escape suppressive effects of anti-retroviral drugs. During cell-to-cell transmission of HIV, a dynamic series of events ensues at the virological synapse that promotes viral dissemination. Cell-to-cell transmission involves various types of cells of the immune system and this mode of transmission has been shown to have an important role in sexual and mother-to-child transmission of HIV and spread of HIV within the central nervous system and gut-associated lymphoid tissues. There is also evidence that cell-to-cell transmission of HIV occurs between thymocytes and renal tubular cells. Herein, following a brief review of the processes involved at the virological synapse, evidence supporting the role for cell-to-cell transmission of HIV in the maintenance of the HIV reservoir will be highlighted. Therapeutic considerations and future directions for this area of research will also be discussed.

P2X-selective purinergic antagonists are strong inhibitors of HIV-1 fusion during both cell-to-cell and cell-free infection

Human immunodeficiency virus type 1 (HIV-1) infection is chronic and presently still incurable. Antiretroviral drugs effectively suppress replication; however, persistent activation of inflammatory pathways remains a key cause of morbidity. Recent studies proposed that purinergic signaling is required for HIV-1 infection. Purinergic receptors are distributed throughout a wide variety of tissue types and detect extracellular ATP as a danger signal released from dying cells. We have explored how these pathways are involved in the transmission of HIV-1 from cell to cell through virological synapses. Infection of CD4+ T lymphocytes with HIV-1 in the presence of an inhibitor of P2X receptors effectively inhibited HIV-1 infection through both cell-free and cell-to-cell contact in a dose-dependent manner. Inhibition of direct cell-to-cell infection did not affect the formation of virological synapses or the subsequent cell-to-cell transfer of HIV-1. During both cell-free and cell-to-cell CD4+ T lymphocyte infection, purinergic antagonists blocked infection at the level of viral membrane fusion. During cell-to-cell transmission, we observed CXCR4 colocalization with the newly internalized virus particles within target lymphocytes and found that the purinergic antagonists did not impair the recruitment of the coreceptor CXCR4 to the site of Gag internalization in the target cell. In a screen of a library of purinergic antagonists, we found that the most potent inhibitors of HIV-1 fusion were those that target P2X receptors, while P2Y-selective receptor antagonists or adenosine receptor antagonists were ineffective. Our results suggest that P2X receptors may provide a therapeutic target and that purinergic antagonists may have potent activity against viral infection of CD4+ T lymphocytes by both cell-free and cell-to-cell transmission.

IMPORTANCE

This study identifies purinergic antagonists to be potent inhibitors of HIV-1 cell-free and cell-to-cell-mediated infection and provides a stepwise determination of when these compounds inhibit HIV-1 infection. These data provide a rationale for the development of novel antiretroviral therapies that have a dual role in both direct antiviral activity and the reduction of HIV-associated inflammation. Purinergic antagonists are shown here to have equivalent efficacy in inhibiting HIV infection via cell-free and cell-to-cell infection, and it is shown that purinergic receptors could provide an attractive therapeutic anti-HIV target that might avoid resistance by targeting a host signaling pathway that potently regulates HIV infection. The high-throughput screen of HIV-1 fusion inhibitors further defines P2X-selective compounds among the purinergic compounds as being the most potent HIV entry inhibitors. Clinical studies on these drugs for other inflammatory indications suggest that they are safe, and thus, if developed for use as anti-HIV agents, they could reduce both HIV replication and HIV-related inflammation.

Which Antibody Functions are Important for an HIV Vaccine?

HIV antibody (Ab) functions capable of preventing mucosal cell-free or cell-to-cell HIV transmission are critical for the development of effective prophylactic and therapeutic vaccines. In addition to CD4(+) T cells, other potential HIV-target cell types including antigen-presenting cells (APCs) (dendritic cells, macrophages) residing at mucosal sites are infected. Moreover, the interactions between APCs and HIV lead to HIV cell-to-cell transmission. Recently discovered broadly neutralizing antibodies (NAbs) are able to neutralize a broad spectrum of HIV strains, inhibit cell-to-cell transfer, and efficiently protect from infection in the experimentally challenged macaque model. However, the 31% protection observed in the RV144 vaccine trial in the absence of detectable NAbs in blood samples pointed to the possible role of additional Ab inhibitory functions. Increasing evidence suggests that IgG Fcγ receptor (FcγR)-mediated inhibition of Abs present at the mucosal site may play a role in protection against HIV mucosal transmission. Moreover, mucosal IgA Abs may be determinant in protection against HIV sexual transmission. Therefore, defining Ab inhibitory functions that could lead to protection is critical for further HIV vaccine design. Here, we review different inhibitory properties of HIV-specific Abs and discuss their potential role in protection against HIV sexual transmission.

HIV-1 vaginal transmission: cell-free or cell-associated virus?

The vast majority of new HIV infections in male-to-female transmission occurs through semen, where HIV-1 is present in two different forms: as free and as cell-associated virus. In the female lower genital tract, semen mixes with female genital secretions that contain various factors, some of which facilitate or inhibit HIV-1 transmission. Next, HIV-1 crosses the genital epithelia, reaches the regional lymph nodes, and disseminates through the female host. Cervico-vaginal mucosa contains multiple barriers, resulting in a low probability of vaginal transmission. However, in some cases, HIV-1 is able to break these barriers. Although the exact mechanisms of how these barriers function remain unclear, their levels of efficiency against cell-free and cell-associated HIV-1 are different, and both cell-free and cell-associated virions seem to use different strategies to overcome these barriers. Understanding the basic mechanisms of HIV-1 vaginal transmission is required for the development of new antiviral strategies to contain HIV-1 epidemics.

Unique functional properties of conserved arginine residues in the lentivirus lytic peptide domains of the C-terminal tail of HIV-1 gp41

A previous study from our laboratory reported a preferential conservation of arginine relative to lysine in the C-terminal tail (CTT) of HIV-1 envelope (Env). Despite substantial overall sequence variation in the CTT, specific arginines are highly conserved in the lentivirus lytic peptide (LLP) motifs and are scarcely substituted by lysines, in contrast to gp120 and the ectodomain of gp41. However, to date, no explanation has been provided to explain the selective incorporation and conservation of arginines over lysines in these motifs. Herein, we address the functions in virus replication of the most conserved arginines by performing conservative mutations of arginine to lysine in the LLP1 and LLP2 motifs. The presence of lysine in place of arginine in the LLP1 motif resulted in significant impairment of Env expression and consequently virus replication kinetics, Env fusogenicity, and incorporation. By contrast, lysine exchanges in LLP2 only affected the level of Env incorporation and fusogenicity. Our findings demonstrate that the conservative lysine substitutions significantly affect Env functional properties indicating a unique functional role for the highly conserved arginines in the LLP motifs. These results provide for the first time a functional explanation to the preferred incorporation of arginine, relative to lysine, in the CTT of HIV-1 Env. We propose that these arginines may provide unique functions for Env interaction with viral or cellular cofactors that then influence overall Env functional properties.

The immunological synapse: the gateway to the HIV reservoir

A major challenge in the development of a cure for human immunodeficiency virus (HIV) has been the incomplete understanding of the basic mechanisms underlying HIV persistence during antiretroviral therapy. It is now realized that the establishment of a latently infected reservoir refractory to immune system recognition has thus far hindered eradication efforts. Recent investigation into the innate immune response has shed light on signaling pathways downstream of the immunological synapse critical for T-cell activation and establishment of T-cell memory. This has led to the understanding that the cell-to-cell contacts observed in an immunological synapse that involve the CD4⁺ T cell and antigen-presenting cell or T-cell–T-cell interactions enhance efficient viral spread and facilitate the induction and maintenance of latency in HIV-infected memory T cells. This review focuses on recent work characterizing the immunological synapse and the signaling pathways involved in T-cell activation and gene regulation in the context of HIV persistence.

Protease inhibitors effectively block cell-to-cell spread of HIV-1 between T cells

BACKGROUND

The Human Immunodeficiency Virus type-1 (HIV-1) spreads by cell-free diffusion and by direct cell-to-cell transfer, the latter being a significantly more efficient mode of transmission. Recently it has been suggested that cell-to-cell spread may permit ongoing virus replication in the presence of antiretroviral therapy (ART) based on studies performed using Reverse Transcriptase Inhibitors (RTIs). Protease Inhibitors (PIs) constitute an important component of ART; however whether this class of inhibitors can suppress cell-to-cell transfer of HIV-1 is unexplored. Here we have evaluated the inhibitory effect of PIs during cell-to-cell spread of HIV-1 between T lymphocytes.

RESULTS

Using quantitative assays in cell line and primary cell systems that directly measure the early steps of HIV-1 infection we find that the PIs Lopinavir and Darunavir are equally potent against both cell-free and cell-to-cell spread of HIV-1. We further show that a protease resistant mutant maintains its resistant phenotype during cell-to-cell spread and is transmitted more efficiently than wild-type virus in the presence of drug. By contrast we find that T cell-T cell spread of HIV-1 is 4-20 fold more resistant to inhibition by the RTIs Nevirapine, Zidovudine and Tenofovir. Notably, varying the ratio of infected and uninfected cells in co-culture impacted on the degree of inhibition, indicating that the relative efficacy of ART is dependent on the multiplicity of infection.

CONCLUSIONS

We conclude that if the variable effects of antiviral drugs on cell-to-cell virus dissemination of HIV-1 do indeed impact on viral replication and maintenance of viral reservoirs this is likely to be influenced by the antiviral drug class, since PIs appear particularly effective against both modes of HIV-1 spread.

High-multiplicity HIV-1 infection and neutralizing antibody evasion mediated by the macrophage-T cell virological synapse

Macrophage infection is considered to play an important role in HIV-1 pathogenesis and persistence. Using a primary cell-based coculture model, we show that monocyte-derived macrophages (MDM) efficiently transmit a high-multiplicity HIV-1 infection to autologous CD4(+) T cells through a viral envelope glycoprotein (Env) receptor- and actin-dependent virological synapse (VS), facilitated by interactions between ICAM-1 and LFA-1. Virological synapse (VS)-mediated transmission by MDM results in high levels of T cell HIV-1 integration and is 1 to 2 orders of magnitude more efficient than cell-free infection. This mode of cell-to-cell transmission is broadly susceptible to the activity of CD4 binding site (CD4bs) and glycan or glycopeptide epitope-specific broadly neutralizing monoclonal antibodies (bNMAbs) but shows resistance to bNMAbs targeting the Env gp41 subunit membrane-proximal external region (MPER). These data define for the first time the structure and function of the macrophage-to-T cell VS and have important implications for bNMAb activity in HIV-1 prophylaxis and therapy. IMPORTANCE The ability of HIV-1 to move directly between contacting immune cells allows efficient viral dissemination with the potential to evade antibody attack. Here, we show that HIV-1 spreads from infected macrophages to T cells via a structure called a virological synapse that maintains extended contact between the two cell types, allowing transfer of multiple infectious events to the T cell. This process allows the virus to avoid neutralization by a class of antibody targeting the gp41 subunit of the envelope glycoproteins. These results have implications for viral spread in vivo and the specificities of neutralizing antibody elicited by antibody-based vaccines.